This invention relates to an electrographic printing or copying machine having a simplified paper path. More particularly, the present invention relates to an electrographic printing or copying machine which employs a vertically mounted photoconductive belt assembly having a simplified paper path whereby an electrostatic latent image is developed on the underside of the copy sheet.
In the process of electrographic or xerographic printing, a photoconductive member is employed to record an image. The photoconductive member, which may be in the form of a belt or a drum, is charged to a substantially uniform potential to sensitize its surface. In the case of a copying machine, the charged portion of the photoconductive surface is exposed to a reflected light image of an original document to be reproduced. The light image is recorded as an electrostatic latent image on the photoconductive member corresponding to the informational areas contained on the original document.
In the case of a printer connected to a computer, a similar process is used to record information on the photoconductive member. The charged portion of the photoconductive surface is exposed to a light image, the shape of which is controlled by input signals from the computer. For example, a laser or an LED array receiving input signals from the computer illuminates the photoconductive member with a light image of a particular shape. Here too, an electrostatic latent image corresponding to the desired informational areas is recorded on the photoconductive member.
After recording the electrostatic latent image on the photoconductive member, the latent image is developed by bringing a developer material or toner into contact with it. The developer material is attracted to the electrostatic latent image and forms a powder image on the photoconductive member corresponding to the electrostatic latent image. The powder image is subsequently transferred to a sheet of recording medium, such as a sheet of paper. Thereafter, the powder image is permanently affixed to this sheet in image configuration by a variety of methods, such as by fusing.
The above-mentioned operations may be carried out by arranging a number of stations in sequence about the photoconductive member. Thus, the photoconductive member is usually surrounded in sequence by a charging station, an imaging station, a developing station, and a transfer station. A discharging station and a cleaning station are also arranged about the photoconductive member to ready it for use again.
An example of such an electrographic printing apparatus is disclosed in allowed application Ser. No. 700,813, filed Feb. 11, 1985, now U.S. Pat. No. 4,664,507. The electrographic printer/copier described in that patent application employs a photoconductive belt assembly in the form of a disposable cassette which is described and claimed in allowed application Ser. No. 718,947, filed Apr. 2, 1985, now U.S. Pat. No. 4,657,309. The printer/copier described in application Ser. No. 700,813 also employs the combined developing and cleaning unit which is the subject of allowed application Ser. No. 718,946, filed Apr. 2, 1985, now U.S. Pat. No. 4,639,116. All of these aforementioned patent applications are assigned to the present assignee and all are incorporated herein by reference.
The electrographic printer/copier described in application Ser. No. 700,813 has a simplified paper path permitting access from the top of the machine. In that electrographic printer/copier, the cassette containing the photoconductive belt is mounted vertically within the machine and a latent image is developed on the underside of the copy sheet as it passes over and comes in contact with the top of the photoconductive belt assembly.
That electrographic printing machine requires two rotations of the photoconductive belt per copy produced. It is capable of producing about 12 copies per minute. During the first rotation of the photoconductive belt, the belt is uniformly charged and a latent image is generated by means of an optical print head on the surface of the photoconductive belt. The latent image thus formed is developed by the deposition of toner from the developer/cleaning unit operating in the develop mode. The belt then enters the transfer region wherein the developed image is transferred to the underside of the paper or other copy material. In the transfer region, a transfer unit generates an electrical field which attracts the toner from the photoconductive belt to the underside of the paper. This completes the first rotation of the belt as the paper travels to a fuser unit and is discharged into the output tray.
During the next revolution of the belt, the belt is prepared for making the next copy. The main charging unit and the optional print head are disabled while an erase lamp is activated and the developer/cleaner unit is switched to the clean mode. Thus, as the belt continues to rotate following image transfer, the photoconductive belt is discharged by an erase lamp and the excess toner is removed using a conventional electrostatic process by the developer/cleaner unit. The belt is thereby readied for the next copy in the next revolution of the belt.
In order to increase the output of the above-described electrographic printer/copier, it is desirable to modify the machine by making it a "one-pass" device. That is, it would be desirable for the electrographic printer/copier to require only a single rotation of the photoconductive belt per copy produced. In this way, the output of the machine would be doubled.